Cancer prevention is an important strategy in cancer control and it consists of primary prevention and secondaryprevention. Major avoidable or manageable risk factors for cancer identified from previous studies are tobacco,diet and infection. Some cancer could be prevented by controlling those risk factors. In Japan screenings for gastriccancer, cervical cancer, lung cancer, breast cancer and colo-rectal cancer have been widely conducted under the Lawof Health Maintenance for the Aged. In planning and evaluating cancer control activities in Japan, it was considereduseful to estimate the potential of primary and secondary prevention of cancer. The author estimated the potentialof cancer prevention in Japan twice previously in 1990 and 1999. In this paper the potential of cancer prevention inJapan was re-estimated by using a different method and more recent data. From the present study it was estimatedthat about 25% of cancer occurrence could be prevented by control of smoking, diet and infection, about 9-15% ofcancer deaths could be prevented by cancer screening, and about 6- 10 % of cancer deaths could be prevented byapplication of the state-of-the art diagnosis and treatment of cancer, altogether about 40-50% of cancer occurrences/deaths could be prevented if all possible measures for cancer prevention are applied to the general public and cancerpatients in Japan.